Introduction

The adage “time is money” is certainly relevant to many industries as it relates to building renovations and the cost of downtime, interruptions to services or inconveniences to customers. To the casino industry, the phrase is even more literal. Any interruptions to revenue generating operations come at great expense to the owner.

That is why the owner of Resorts Casino and Perini Building Company -- the largest builder of hotels and casinos in the United States and general contractor for new construction and renovation projects at Resorts -- chose to create a team that could bring together both engineering and construction expertise for a critical path concrete repair and strengthening project. They assembled a team that included Structural Group – one of the nation’s leading specialty contractors in concrete repair -- and Lochsa Engineering -- one of the premier design groups for casino construction and renovations.

As the first casino on the East Coast when it opened in 1978, Resorts Atlantic City has set the standard for casino gaming and entertainment. Situated on 11 acres of land with approximately 310 feet of boardwalk frontage overlooking the Atlantic Ocean, the 100,000-square foot casino features action 24-hours-a-day at more than 70 tables and nearly 3,000 slot machines. The original structure, constructed in the 1920s, consisted of two twin towers which, over the years, have undergone various renovations.

Recently, Perini Building Company demolished one of the two original towers and constructed a new, state-of-the-art hotel tower in its place. The final phase of this construction project was to connect the original hotel tower, lobby, check-in area and casino with the new hotel’s lobby by creating a new promenade between them. The access would allow customers to easily reach all casino services without having to walk outside between the two towers.

With the new tower open, the need for this access was immediate. However, because of differences in the slab elevations of the floors in the two towers, the new promenade would need to be inclined to compensate for these differences. To resolve the situation, an existing intermediate slab at the old tower had to be removed in order for the new concourse to pass through from the lower elevation at the old tower to the higher one at the new tower. Although demolition of the intermediate slab was not complex, the change of the structural condition resulted in columns that span two floors high. A concern with the old columns’ strength and their ability to support the original building loads required further evaluation. A second challenge related to a condition that was not discovered until construction of the promenade had begun. The new walkway was designed to be supported through a series of supports resting on the original base slab below. The base slab was thought to be a slab on grade. As construction progressed, an entrance hatch was discovered on the wall in the area adjacent to the original base slab. Since this was thought to be a foundation wall, the discovery of an access hatch was very confusing. After further investigation, it was discovered that this part of the base slab – a part originally considered a slab on grade – was actually an elevated slab that had an old steam tunnel under it. Worse yet, it was discovered that the existing structural beams supporting the slab in this steam tunnel had never been maintained and had severely deteriorated over time to the point that they would not be able to carry the new loads. In fact, the condition of the beams was such that serious doubts existed about their ability to support even the current loads. Temporary shoring was installed immediately to support the slab until remediation and strengthening plans could be established.

The team also discovered a similar condition of severe deterioration in beams, slabs, and columns in a below-grade room called the Grease Recovery Unit (GRU) room. The GRU was adjacent to the steam tunnel and functioned as a processing room for the massive amounts of grease created daily by the many hotel restaurants. Shutting it down for structural repairs in any way would cause major disruptions.

Because these three areas needed to be repaired and/or strengthened before any portion of the new concourse could be completed, a timely evaluation, repair strategy and installation process was essential. Recognizing the importance of proper and timely restoration, the owner tasked Perini Building Company to oversee the project, beginning with assembly of a team to develop strategies to repair and/or strengthen the deteriorated structural concrete elements. As such, the scope of the repair project was re-defined as:

Strengthen the columns in the new promenade to prevent buckling failure

Understanding Structural Repair vs. Strengthening

In order to gain an accurate sense of the work at-hand, it is important to first understand the difference between repairing and strengthening concrete structures. Structural repair describes the process of in-kind reconstruction of concrete members to bring them back to their original capacity and condition. In these cases, the cause of the deterioration is determined, deteriorated materials are removed and repair materials designed to extend the structure’s useful life are selected and installed. Structural strengthening, on the other hand, describes the process of upgrading existing concrete members to improve their load carrying capacity for additional loads or stresses.

For concrete strengthening projects, the design strategy must take into account that every member in the structure is already carrying a share of the existing loads. The effects of strengthening or removing any portion of a structural member -- whether permanent or temporary -- must be carefully analyzed to determine its influence on the global behavior of the structure. Failure to do so can easily overstress members in the vicinity of the modified ones. On the installation side, the construction sequence must address access and constructability issues as well as phasing of repairs and temporary shoring. Issues related to the effects of using repair materials that are not typical to new construction must be evaluated. Compatibility of new and existing materials, curing time and ensuring adequate load sharing must all be considered. Also, with an occupied and operating structure, dust and noise control are critical concerns.

Although cosidered long-lasting and durable, buildings constructed using reinforced concrete, such as the Resorts Casino, have a finite service life. When exposed to harsh environments, chlorides, de-icing salts and chemicals, these structures will naturally experience significant deterioration – typically in the form of cracks, steel corrosion and concrete spalling. One of the most severe and widespread deterioration mechanisms in concrete is the internal damage created when external chlorides seep into the concrete and corrode the embedded reinforcing steel. Corrosion problems are initiated by the corrosion by-product (rust) that expands up to eight times the original steel size. This expansion creates high internal pressures that eventually cause the concrete over the rebar to crack and then completely spall off. The corrosion process also results in a reduction of the effective area of steel rebar and, therefore, reduced structural capacity of the deteriorated member. If not addressed at early stages, corrosion will continue to grow rapidly – ultimately creating a safety issue due to falling concrete and loss of strength.

Bigger is Not Better – Column Strengthening Approach

According to Bill Segal, General Superintendent on the Resorts Casino project for Perini Building Company, during the demolition phase, the existing intermediate slab at the old tower was removed to allow for the creation of the new promenade slab. After removal, a structural engineer with Lochsa Engineering conducted a survey of the existing condition and determined that the columns were not made of reinforced concrete as originally thought but rather consist of concrete-encased steel “W” shapes. The concrete jackets on these columns had minimal reinforcement and were apparently used to provide fire protection rather than a load bearing component. As a result, it was decided that a structural evaluation was necessary to ensure that the now double-height steel columns were adequate to support the building loads. Ignoring the concrete jacket, analysis showed that the column capacity was controlled by the column buckling strength and was overstressed because of the change in the unsupported column height (10 feet to 20 feet) caused by the removal of the original slab.

Traditional methods for strengthening columns involve the use of steel and concrete in the form of a cast-in-place reinforced concrete jacket around the column installed to improve the buckling resistance of the columns. However, this option was not considered viable because it would increase the overall column size and reduce the width of the new corridor below code requirements for safe egress.

Accordingly, another strengthening option using very thin, high-strength, carbon fiber sheets (commonly known as fiber reinforced polymers or FRP) was considered for this strengthening application. These paper-thin sheets would be externally bonded to the concrete columns using epoxy adhesive in both horizontal (hoop) sheets and continuous vertical sheets to create a bi-directional composite jacket that would effectively reinforce the existing concrete jacket and utilize it in resisting buckling forces. While adding less than one-inch to the column dimensions, the externally bonded composites jacket effectively upgraded the column with minimal impact on the schedule, building operations and egress requirements. As a non-corrosive material, FRP composites are quick and easy to install and are easier to conceal. Structural Group was contracted to work with the structural engineer to design and install a carbon fiber reinforced polymer (CFRP) system on the columns to provide the necessary strength for eight columns and the floors above this 8,000-square-foot area.

As with any ther externally-bonded system, the surface preparation and bond between the FRP system and the existing concrete is very critical. On this project, proper installation criteria was achieved by sandblasting the entire surface, and patching holes and deviations on the faces of columns using a manufacturer recommended epoxy putty filler. All column corners were rounded to at least a one half-inch radius to prevent stress concentrations that can cause premature composite failure. FRP installation was achieved by applying a layer of epoxy adhesive to the prepared surface, installing the carbon fiber reinforcement into the wet epoxy, and then applying a second coat of the epoxy adhesive. This process was repeated for additional layers of FRP. As a quality control measure, pull-off tests were performed periodically to ensure that the minimal bond strength of 200 psi was achieved as specified by ACI 440. Witness panels of the composite system were fabricated in the field and send to an independent lab for testing to confirm FRP tensile properties.

Two columns that required strengthening were inside the Fire Control Room (FCR), which monitors the fire alarm security systems for the entire facility. To keep the FCR in operation while maintaining all safety and code requirements, two-hour rated falsework walls were constructed above the FCR to isolate the work from operational equipment. One column, still attached to the original façade, could not be wrapped with FRP because access to all four faces of the column was not possible. This column had an existing transverse beam attached to it at mid-height in one direction making column buckling only a concern in the other direction. To resolve this, a new lateral steel support beam was provided to the column to brace it in the orthogonal direction to that of the existing transverse beam. Because a very limited work area was available, the new bracing, which consisted of a large steel beam, was fabricated in three bolt-together sections and assembled in place. The new beam was installed at column mid-height and attached to the adjacent column -- acting as a horizontal brace to prevent buckling.

It’s Getting Hot in Here: Concrete Repairs in the Steam Tunnel

With the column strengthening under control, attention was directed to the severe concrete deterioration in the steam tunnel and the GRU. In many of the beams, slabs and columns, the steel reinforcement was fully exposed or completely deteriorated. Not only would these members require repair, they would also have to be strengthened to meet today’s building codes.

In the steam tunnel, the working conditions and the upgrade design strategy presented unique challenges. Access to the beams was difficult at best. With an access hole of approximately three feet by two feet - the work area was considered a confined space. Key safety issues needed to be addressed and, in addition to having a dozen super-heated high-pressure steam lines, the working area in the tunnel was very tight – giving new meaning to “back-breaking work.”

The design and construction team developed a creative approach to address the design requirements, limited access, tight working conditions, low headroom, air quality and ventilation issues. To support the floor above during removal and enlargement of the beams, the entire ceiling (base slab) was shored. Also, the floor area over the steam lines was fully decked to protect both the workers and the steam lines during construction – giving the workers only 36 inches of working height. Workers, equipment and demolition debris would generally be transported in and out with low profile carts. In order to alleviate egress challenges, a second entrance was created at the other end of the tunnel. Also, through an engineered solution using forced air and airflow exchanges (approximately 30 cubic feet per hour), it was possible to reduce the normally 90+ degree Fahrenheit temperatures inside the steam tunnel to the high 70s.

In order to develop an adequate strengthening strategy, and because no structural drawings of the original construction were available, a comprehensive field investigation of the existing beams was needed. In order to determine the size and layout of the bottom steel reinforcement, the soffit of every beam was fully chipped at quarter span to expose the reinforcement. The quarter span chipping location was selected because it was determined that it would have the least combination of bending and shear forces thus reducing the risk of failure.

Field investigation of the existing reinforcing bars revealed that all ten beams supporting the tunnel ceiling slab had completely different rebar configurations – giving each a different in-place capacity. This was related to the poor quality control of the original construction during which the actual depth and spacing of the bases was not properly controlled. For example, in some instances, the steel bars were placed adjacent to each other with no spacing in between. In other instances, bars that should have been two inches from the bottom of the beam where found to be as high as six inches from the bottom. Accordingly, all of the beams in the steam tunnel required both flexural and shear upgrade to carry the new loads and all were deficient in strength and in a state of severe deterioration. To strengthen the beams, it was decided to employ an enlargement technique involving a cast-in-place reinforced concrete jacket that would be installed on each existing beam. The challenge faced from a design perspective was defining the existing capacity of each beam as a reference for the level of the required upgrade. For speed and simplicity, the design team decided that the new jacket would be designed to disregard the existing steel reinforcement and would rely completely on the new jacket reinforcement for strength to support the new loads. This plan addressed the upgrade in all the beams without the need to generate 10 separate designs – a common strengthening strategy. This also addressed concerns with uncertainties related to the effectiveness of the existing steel reinforcement considering the observed level of deterioration.

To start the repair, temporary shoring was placed under the members and all deteriorated concrete was removed with small chipping hammers. The exposed concrete and steel were treated with abrasive blasting to clean the steel bars and to create open surface pores on the concrete that would promote a strong bond with the new material. Creating a rough surface with an open-pore structure would also guarantee a composite behavior between the old and new concrete. U-stirrups for shear were doweled in every five inches along the beam and new bottom steel reinforcement was placed. The walls supporting the beams were chipped back around all the beams two to three inches to key the new enlargement into the wall. In addition, longitudinal steel bars were doweled into the concrete wall to provide an additional shear transfer mechanism. The enlargement area was formed such that the beams would be enlarged by five inches on all three sides. Specially designed formwork was then placed on all of the beams so that the new concrete could be sequentially pumped into all 10 beams on the same pour. Placing the concrete using the form-and pump technique is one of the most effective methods of ensuring composite behavior of the new enlargement with the existing member. Combined with adequate surface preparation, this concrete placement method ensures that the neat paste of the new concrete is “forced” into the pores of the prepared concrete surfaces by the pressure of the pumping and creates a mechanical bond with the old concrete and hence composite behavior.

Self-consolidating concrete, or SCC, was selected as the repair material of choice because it could be pumped the long distance from the concrete truck to the repair area and it would flow easily in and around the high concentration of reinforcement in the beam.

Plexiglass windows were created in the formwork at the end of each beam to visually inspect the flow of concrete and ensure that it was pumped to capacity. At the other end of the beam, a connection port for the concrete line was installed. Because the SCC repair material easily pumped and flowed, Structural Group used only a single port on the formwork – allowing pumping through the form for many feet without segregation. With the low ceiling height, this feature was beneficial because it eliminated many connections – reducing the amount of work and workers in the tunnel during placement.

GRU Room Concrete Repair and Strengthening

This below sea level room contained the grease recovery and processing system for all of the casino restaurants. The amount of grease processed every day is far more than one would think and large process tanks are required. The concrete ceiling at the GRU room consists of a two-way slab and beam construction that, over many years, had deteriorated to a point of structural concern. A constant combination of high temperatures and humidity in the room over time had caused corrosion of the reinforcement and significant concrete spalling. Further, three very large columns – each supporting more than 12 floors of the building – were deteriorated and did not meet the current design codes in terms of lateral reinforcement.

This general area was so congested with equipment and piping that it was difficult to actually see the beams and slabs above. There were in excess of 100 pipe-hangers supporting approximately two tons of Mechanical/Electrical/Plumbing equipment in a 30-foot-by-15-foot area. Clear access to the concrete repairs would have required removing the process equipment and associated piping – causing major disruptions to all of the restaurants. Instead the team developed an elaborate shoring plan to support the equipment and piping which allowed the entire beam and slab system to be removed and replaced without any operational downtime.

Once the original ceiling structure was completely demolished, formwork was placed and a new two-way slab and beam system was cast with new hangers and supports for the piping and the equipment. Next, reinforced concrete enlargements were designed and installed to repair and strengthen the columns. First, 6 inches of concrete were removed on all four faces of the 48-inch square columns. New vertical steel, doweled into the foundation, was installed along the faces of the column and new closed stirrups were placed around the columns before the columns were formed. Again, SCC was used to enlarge the columns. All SCC used on these repairs was specifically formulated to meet strict flow and curing time requirements. All concrete mixes aimed at achieving high early strength to expedite the construction process and keep up with the very tight construction schedule.

Open For Business

Although the repaired areas were completely closed off to pedestrians, the owner wanted the Casino to stay operational during the reconstruction. Safety and minimizing interruptions and noise were paramount. All demolition, drilling and pumping operations adhered to stringent starting times for any noise-producing work. Brett Schneider, Structural Group Superintendent, said key to success for this fast-track project was extensive pre-planning between the owner, engineer and contractors.

“Prior to starting this project in September, the team had more than 100 man-hours pre-planning the work,” Schneider said. “This allowed an outline to be created for each area of work and what challenges each area presented. We also scheduled a two-day safety training and orientation seminar prior to mobilizing the project. This allowed for a successful integration of our employees with the local trade unions.”

On the technical side, engineering pre-planning was also a key to success for this project. Several meetings were held before and throughout to predict possible challenges and repair option scenarios and develop solutions before delays occurred. As with any successful strengthening project, the Resort upgrade project required a balance between technical issues such as sound engineering concepts, temporary shoring and creating composite behavior of repair and strengthening solutions with issues related to contractibility and scheduling of these solutions. It was also important to consider solutions and processes that would address non-technical issues such as access, safety, constructability, economics and aesthetics.

This fast-track project was completed ahead of schedule and was the result of a very open and cooperative relationship between the contractors, the engineer and the owner. The completion of Structural Group’s repairs allowed Perini to move forward with the remaining architectural changes and the Grand Opening of the new concourse occurred in January 2006, ahead of schedule and on budget.

Concrete Repair and Maintenance Illustrated

Written by Peter H. Emmons, CEO of STRUCTURAL and an international authority on concrete repair.